Journal of the Ceramic Society of Japan 105 [6] 482-485 (1997) Paper

Structure Determination of New Layered Perovskite Compound, NaLaTa2O7,

Synthesized by Ion-Exchange Reaction

Kenji TODA, Kazuyoshi UEMATSU* and Mineo SATO*Department of Active Material Chemistry, Graduate School of Science and Technology, Niigata U n i v e r s i t y ,

8050, Ikarashi 2-nocho, Niigata-shi 9 5 0 - 2 1* Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata U n i v e r s i t y , 8050, Ikarashi 2-nocho, Niigata-shi 9 5 0 - 2 1

イ オ ン 交 換 反 応 に よ り合 成 した 新 し い 層 状 ペ ロ ブ ス カ イ ト化 合 物NaLaTa2O7の 構 造 決 定

戸 田健司 ・上松 和義*・ 佐藤峰夫*

新潟大学大学院自然科学研究科, 950-21新 潟市五十嵐二の町8050* 新潟大学工学部化学システム工学科, 950-21新 潟市五十嵐二の町8050

A new layered perovskite compound, NaLaTa2O7, was synthesized by ion-exchange reaction of the rubidium compound, RbLaTa2O7. The crystal structure of the compound was determined by the Rietveld analysis for

powder X-ray diffraction pattern. The crystal structure is analogous to that of the corresponding niobate compound, NaLaNb2O7. This layered perovskite compound is a new member of the Dion-Jacobson series with n=2 for the general formula M[An-1BnO3n+1] (M=alkali metals).

[Received October 31, 1996; Accepted March 11, 1997]

Key-words: Layered perovskite, Ion-exchange reaction, Hydration, Crystal structure, Sodium lanthanum tantalate

1. IntroductionThe discovery of new solid state compounds with unique

structures and physical properties is related to the development in synthesis techniques such as soft chemical routes. Several examples are reported by Gopalakrishnan.1) Ion-exchange reactions in inorganic solids can be used to provide metastable phases. The ion-exchangeable layered perovskites have enough spaces to allow ion transport. The interlayer positions of ion-exchangeable layered perovskites can be occupied by alkali ions. These compounds exhibit a variety of chemical properties such as ion-exchange and intercalation reactions. There are two series of ion-exchangeable layered perovskites; one is Ruddlesden-Popper series2)-5) with the general formula M2[An-1BnO3n+1] (M=alkali metal, B=Ti) and the other is Dion-Jacobson series6),7) with M[An-1BnO3n+1] (M=alkali metal, B=Nb).

Recently, we prepared a new ion-exchangeable layered perovskite compound, RbLaTa2O7, and determined the crystal structure of its derivative, LiLaTa2O7.8) These layered perovskite compounds are new members of the Dion-Jacobson series with n=2. In the course of the study for this compound, we have found that the rubidium ions in the RbLaTa2O7 are readily exchanged with sodium ions to give a new layered perovskite compound, NaLaTa2O7.

In this paper, we report the synthesis of new layered perovskite compound, NaLaTa2O7 and its hydrate. The crystal structures of both compounds were determined by the Rietveld analysis for powder X-ray diffraction patterns. We discuss the stacking feature of the perovskite layer on the basis of these structure refinement.

2. ExperimentalThe parent compound, RbLaTa2O7, was prepared by a

conventional solid-state reaction. The starting material was a mixture of rubidium carbonate, lanthanum oxide and tantalum oxide. An excess amount of rubidium carbonate (50mol%) was added to compensate for the loss due to the volatilization of rubidium component. The pellet was placed in an alumina crucible and then heated at 1373K for 4h in air. Ion exchanged compound, NaLaTa2O7, was prepared by the ion-exchange reaction from the parent compound.

The ion exchange of sodium compound was carried out by

reacting the parent compounds with molten NaNO3 at 673K

for 24h. After the reaction, the precipitates of the products

were collected, washed with distilled water and air-dried at

room temperature. The completion of the ion-exchange

reaction was confirmed by X-ray diffraction (XRD) pat

tern.

Powder XRD patterns were recorded on a Rigaku RAD

rA diffractometer equipped with a curved crystal graphite

monochromater using CuKƒ¿ radiation at 298K. The data

were collected by a step-scanning mode in the 2Į range of

5-100•‹ with a step width 0.02•‹ and a step time 4s. Since the

NaLaTa2O7 compound was strongly hygroscopic upon ex

posure to the atmosphere, the powder XRD pattern of anhy

drous compound was obtained at 573K. Indexing of the

powder XRD pattern obtained was examined with the aid of the computer program CELL.9) Data analysis was carried

out by the Rietveld method, using the RIETAN94 profile

refinement program.10) The reflection peak observed at low

2Į was found to deviate greatly from the calculated peak

profile due to an asymmetric effect of peak shape. Therefore, the reflection peaks below 2ƒÆ=20•‹ were eliminated

from the Rietveld refinement. Thermogravimetric analysis

(TGA) was done using an SSI EXSTAR6000 at a heating

rate of 2K min-1 in air.

3. Results and discussion

3.1 Synthesis of NaLaTa2O7

First, we attempted the solid-state reaction to obtain a

layered perovskite compound, NaLaTa2O7. The correspond

ing niobate compound, NaLaNb2O7, could not be synthe

sized by the solid-state reaction.11) Even the tantalum com

pound, NaLaTa2O7, also is not able to obtained by the solid

state reaction at temperatures ranging from 1273 to 1473K.

The main product is a perovskite phase, NaTaO3, and a

large amount of a second phase, LaTaO4, was present. Chin

cholkar reported that the tolerance factors of the

pyrochlore structure for many ternary oxide systems, alkali

metal-rare earths-pentavalent ions, is close to the values re

quired from the ideal pyrochlore structure.12) Our result is

quite different from that obtained by Chincholkar. The

4 8 2

Kenji TODA et al. Journal of the Ceramic Society of Japan 105 [6] 1997 4 8 3

layered perovskite structure is thermodynamically less sta

ble than the three-dimensional linked perovskite structure

on this trinary system. The layered perovskite compound

NaLaTa2O7, which is probably metastable, could only be ob

tained by the ion-exchange reaction.

3.2 Characterization of hydration

The XRD pattern of NaLaTa2O7•ExH2O shows that

diffraction peaks corresponding to the d-spacings along the

stacking direction of the perovskite layer shift to the lower

side of the diffraction angle during hydration (Fig. 1). This

obviously indicates that the water molecules can be inserted

into the interlayer space. Figure 2 shows TGA data for the

dehydration of the layered perovskite compound,

NaLaTa2O7•ExH2O. The dehydration process of the interlay

er water can be roughly classified into two stages. It is

difficult to isolate a single-phase in the first step. Therefore,

we do not discuss structural characteristics of this com

pound in detail here. The total weight loss of NaLaTa2O7•ExH2O corresponds to 1.9mol of water compared to the anhy

drate, giving an initial composition of NaLaTa2O7•E1.9H2O.

The anhydrate and partially hydrate forms can be easily re

hydrate in a wet atmosphere. The hydration-dehydration

process was found to be reversible.

3.3 Crystal structure of hydrous compound,

NaLaTa2O7•E1.9H2O

The reflection peaks of NaLaTa2O7•E1.9H2O was indexed

with a tetragonal symmetry. The reflection condition found

was h+k+l=2n for hkl reflections, leading to possible

space groups with I-type lattice (I4, I4, I4/m, I422, I4mm,

I4m2, I42m and I4/mmm). The Rietveld refinement was

carried out for all space groups given by the CELL results

in the first refinement stage. An initial structural model was

adopted on the basis of assumption that the main lattice co

nstructs a double perovskite layer analogous to LiLaTa2O7

which has double perovskite layers with displacement of

each layer. The perovskite layer only was first refined. A

difference Fourier map was then generated from the ob

served and calculated structure factors. The most reliable

solution with physically meaningful crystallographic

parameters was finally achieved when adopting I4/mmm space group for NaLaTa2O7•E1.9H2O.

The result of the pattern fitting is shown in Fig. 3. The

structural model for NaLaTa2O7•E1.9H2O is illustrated in

Fig. 4. The crystallographic data finally refined by the

Rietveld refinement are listed in Tables 1 and 2. Although

the hydrous compound gave slightly poor crystallinity com

pared with the parent compound, the R-factors were reasonably converged to acceptable values. The sodium atom

NaLaTa2O7•E1.9H2O is surrounded by four oxygen atoms be

longing to the water molecules. The distance between the

oxygen atoms of water molecules is about 0.275nm. This

distace is close to that found in ice, 0.275nm.13) It is sug

gested that the hydrogen bonds is forming between the four water molecules.

Fig. 1. Powder X-ray diffraction patterns for NaLaTa2O7•E1.9H2O

and NaLaTa2O7. The triangle (•¥) represent the reflection of a sam

ple holder and a furnace attachment.

Fig. 2. TGA curve for NaLaTa2O7•E1.9H2O.

Fig. 3. X-ray powder pattern fitting for NaLaTa2O7•E1.9H2O. The

calculated and observed patterns are shown on the top solid line

and the dots, respectively. The vertical marks in the middle show

positions calculated for Bragg reflections. The trace on the bottom

is a plot of the difference between calculated and observed intensi

ties.

Fig. 4. Structural model of NaLaTa2O7•E1.9H2O.

3.4 Crystal structure of anhydrous compound,

NaLaTa2O7

The indexing for anhydrous compound, NaLaTa2O7, was

also examined. The reflection condition of XRD pattern

found for NaLaTa2O7 was h+k+l=2n for hkl reflections,

giving eight possible space groups with I-type lattice. This condition is identical to that of hydrous compound.

The result of pattern fitting for NaLaTa2O7 is shown in

4 8 4 Structure Determination of New Layered Perovskite Compound, NaLaTa2O7, Synthesized by Ion-Exchange Reaction

Table 1. Crystallographic Data of NaLaTa2O7•E1.9H2O

a) Multiplicity and Wyckoff notation.b) Occupancy.c) Oxygen atom of water molecule.

Table 2. Selected Interatomic Distances for NaLaTa2O7•E1.9H2O

Fig. 5. X-ray powder pattern fitting for NaLaTa2O7. The addition

al rows of Bragg reflection marks corresponding to Pt holder.

Table 3. Crystallographic Data of NaLaTa2O7

a) Multiplicity and Wyckoff notation.

b) Occupancy.

Table 4. Selected Interatomic Distances for NaLaTa2O7

Fig. 5. The crystallographic data for NaLaTa2O7 are listed in Tables 3 and 4. Figure 6 shows the crystal structure of NaLaTa2O7, together with those for LiLaTa2O7, RbLaTa2O7 and KLaTa2O7, as a reference.8),14) All strucures are characterized as a nearly two-dimensional frame

work comprised of two coner-shared TaO6 octahedra. The alkali ions in these layered compounds are located in the interlayer space between the perovskite slabs. The relative arrangement of the adjacent perovskite sheets is dependent on the size of ions existing in the interlayer. The perovskite layer in rubidium compound is stacked up without a displacement. The adjacent perovskite layers of the potassium compound are stacked with a displacement by 1/2 along the only one direction within the layer plane. The potassium

Kenji TODA et al. Journal of the Ceramic Society of Japan 105 [6] 1997 4 8 5

ions are located in the interlayer space and are in a trigonal prismatic coordination. On the other hand, the adjacent perovskite sheets in sodium and lithium ion-exchanged compound are stacked with a displacement by 1/2 along the diagonal direction within the layer plane. The sodium and lithium ions occupy the four-fold sites between the two perovskite layers. The tetrahedral coordination of Na-O found in NaLaTa2O7 is extremely rare in the normal oxides with layered structures. The coordination number of the sodium ions is usually larger than five. The tetrahedral coordination of sodium atoms are probably thermodynamically metastable. This seems to be one of the reason for difficulty of direct synthesis.

LiLaTa2O7 NaLaTa2O7 KLaTa2O7 RbLaTa2O7

Fig. 6. Structural models of MLaTa2O7 (M=Rb, K, Na, Li).

This stacking feature is in contrast to that of titanate compound.4),5),15) In titanate compounds, the relative arrangement of the adjacent perovskite sheets is independent of its ionic size while existing in the interlayer. The lower charge (4+) of the central ion in the titanate compound is compensated by two interlayer monovalent ions, i.e., a high positive charge density state is realized in the interlayer space. On the other hand, the high positive charge (5+) of the central ion in the tantalate compound can lead to a lower charge density state for the interlayer space. The coulomb interaction between the perovskite layer and the interlayer ion is relatively weak, reflecting a variety of stacking features of the adjacent perovskite layer blocks owing to the ionic size of the interlayer ions. The stacking sequences are dependent on the space volume formed between the perovskite layers. The structural features in these compounds are the same as those of the corresponding niobate compounds.11) It is very interesting that the structures of tantalate compounds and niobate compounds are same. Despite of the difference in electron configuration between the tantalum and niobium atoms, the crystal structures of the tantalate compounds are practically unchanged. The niobium and tantalum ions have the same electric charge and almost same ionic radius.16) Structural data indicate that electric charge and ionic radius play an important role in structural properties of these materials.

Figure 7 shows the environment of tantalum atoms in NaLaTa2O7. The tantalum atom is fairly displaced from the center of the octahedron. The oxygen with a fairly shortened distance of Ta-O is located toward the interlayer. Such shortness of the Ta-O distance seems to cause less interaction between the rubidium and perovskite sheet. Therefore, it is possible in this situation to exchange the interlayer ions with other alkali ions. The Ta-O bonds in NaLaTa2O7 are classified into three types: i.e., a very short bond (0.180(9)nm) toward the interlayer, four normal

bonds (0.200(1)nm) linked within the perovskite layer and a long bond (0.2263(9)nm). These bond distances are similar to that of parent compound (0.169(5)nm, 0.1975(6)nm, 0.2285(4)nm). The ion-exchanged compound, NaLaTa2O7, retains the same bond character as that of the parent compound, RbLaTa2O7.8) An important common structural feature for all the series of ion-exchangeable layered perovskite compounds is the presence of extremely short of metal-oxygen bonds in the direction of the interlayer alkali metals between the perovskite layers. 4 ) , 5 ) , 8 ) ,1 1 )

Fig. 7. Environment around tantalum atoms in NaLaTa2O7.

Acknowledgements We are indebted to Mr. H. Minagawa and Mr. T. Iwafune for his help in data collection of X-ray powder diffraction measurements. We are also grateful to Shin-Etsu Chemical Co., Ltd. for providing the rare earth oxide.

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